The present invention relates generally to the field of power generation and, in particular, to the generation of power from stream using a steam turbine system.
Many processes for the generation of power from steam operate using two stages, the first stage involving the production of a gaseous fuel and the second stage involving the use of the fuel to generate steam which is expanded to produce power.
GB-A-1525490 (Klein et al; published on Sep. 20, 1978) discloses a power generation process in which a fuel is partially combusted in the presence of compressed air. A proportion of the heat liberated is used to produce steam from pre-heated water. The combustion gases are then cleaned, freed of H2S, mixed with compressed air and then combusted completely. The resultant combustion gases drive a turbine. The gases leaving the turbine are passed to an off-gas boiler in which steam generated upstream is further heated. The further heated steam is used to drive a steam turbine.
It is known in the art to use hydrocarbon or carbonaceous feedstock to provide fuel for a power generation plant. For example, it is known to convert natural gas to xe2x80x9csynthesis gasxe2x80x9d (a mixture of hydrogen and carbon monoxide). The gaseous fuel is then fed to a power generation plant comprising a gas turbine system, a heat recovery and steam generation system (xe2x80x9cHRSGxe2x80x9d) and a steam turbine system. The fuel gas is combusted in the presence of a compressed oxidant gas such as air or oxygen to form a mass of hot gaseous combustion products. At least some of the heat generated in the combustion may be recovered in the HRSG by generating steam which is then expanded in the steam turbine system to provide power and expanded steam.
The two stages of these processes are usually independent of each other, the first stage simply supplying the fuel for the second stage.
Conventional steam turbine systems use three pressure levels of steam generation with the expanded steam from the highest pressure turbine being reheated before it is introduced to the medium pressure turbine. A typical steam turbine system is shown as part of a typical two-stage power generation process depicted in FIG. 1.
Referring to FIG. 1, a stream 24 of feed air is compressed C-102 and then fed as a stream 27 to a combustion chamber R-108. A stream 23 of pressurized fuel gas comprising predominantly hydrogen is fed to the combustion chamber R-108 where the air and the fuel are combined and burned and a stream 28 of pressurized gaseous combustion products is removed. This product stream 28 is expanded in a gas turbine T-101 to produce power and a stream 29 of lower pressure gaseous combustion products. Optionally, a stream of nitrogen 76 is added to the combustion chamber R-108 thereby increases the power produced by the expander T-101.
The exhaust 29 from the gas turbine T-101 is typically at about 600xc2x0 C. and is cooled to approximately 100xc2x0 C. in the HRSG X-106. A stream 33 of 20xc2x0 C. water at about atmospheric pressure is fed to the HRSG X-106 in which it is heated to 99xc2x0 C. The warmed water stream 77 is then removed from the HRSG and de-aerated in de-aerator 78. The de-aerated water 79 is then divided into three streams 80, 87, 93. The stream 80 is pumped in pump 81 to about 4 atm. (0.4 MPa) to produce a low pressure stream 82 which is vaporized in the HRSG X-106 to produce a stream 83 of saturated steam at a temperature of 144xc2x0 C. that is then fed to a low-pressure stage T-104 of the three-stage steam turbine. The low-pressure turbine T-104 expands the steam and the resultant exhaust stream 84 has a pressure of about 0.04 atm. (4 KPa) and a temperature of about 29xc2x0 C. The exhaust stream 84 is then condensed X-107 to form stream 85 that is then pressurized in pump P-102 to about 1 atm. (0.1 MPa) to form stream 86. Stream 86 is recycled by addition to the HRSG feed water stream 33.
Stream 87 is pumped in pump 88 to about 35 atm. (3.4 MPa) to form a medium pressure stream 89 which is vaporized in the HRSG X-106 to produce a stream 90, 91 of saturated steam at a temperature of about 243xc2x0 C. The stream 91 of medium pressure steam is fed to the medium pressure stage T-103 of the steam turbine where it is expanded to a pressure of about 4 atm. (0.4 MPa). The exhaust stream 92 is then fed to the low-pressure stage T-104 of the steam turbine.
Stream 93 is pumped in pump 94 to about 150 atm. (15 MPa) to form a high pressure stream 95 which is vaporized in the HRSG X-106 to produce a stream 96 of superheated steam at a temperature of about 585xc2x0 C. The superheated steam 96 is then expanded in a high-pressure stage T-102 of the steam turbine to produce a medium pressure stream 97 at about 35 atm. (3.5 MPa). In the prior art process, the medium pressure exhaust stream 97 is then returned to the HRSG X-106 and reheated to about 550xc2x0 C. The reheated medium pressure stream 98 provides a portion of the feed stream 91 for the medium pressure stage T-103 of the steam turbine.
The graph in FIG. 2 depicts a typical cooling curve for a HRSG in combination with a conventional three level steam turbine system in a process according to the flow sheet in FIG. 1. The ideal rate of cooling, represented by the upper line, would be constant thereby maximizing the efficiency of the process. Use of more pressure levels of steam generation would improve the efficiency of the power generation process as the actual cooling curves in the HRSG would match more closely the ideal cooling curve. However, increasing the number of pressure levels in this way would significantly increases the capital, running and maintenance costs of the process. It is the primary objective of this invention, therefore, for provide a modified process that strikes a balance between performance and cost.
It has been found that the primary objective of the invention can be achieved by using the heat generated in an exothermic fuel gas generation process to produce the steam for expansion in the steam turbine system. This significantly improves the efficiency of the overall power generation process. The inventors are not aware of any system in which the high pressure steam vaporisation duty is carried out outside the HRSG.
In particular, power is produced from hydrocarbon fuel gas by a process comprising generating exothermically a first fuel gas. An oxidant gas is compressed to produce compressed oxidant gas. A second fuel gas is combusted in the presence of at least a portion of the compressed oxidant gas to produce combustion product gas, at least a portion of which is expanded to produce expanded combustion product gas. Pre-heated water is at least partially vaporized by heat exchange against at least a portion of the first fuel gas to produce an at least partially vaporized water stream. This water stream is heated by heat exchange against expanded combustion product gas to produce a heated first steam stream at a pressure of from 100 atm. (10 MPa) to 200 atm. (20 MPa). The heated first steam stream is expanded in the highest pressure stage of a steam turbine system having more than one pressure stage to generate power and an expanded steam stream.
The latent heat duty for at least partially vaporising the pre-heated water is provided by the first fuel gas rather than by the expanded combustion product gas. Thermal integration of the process in this way improves significantly the overall thermal efficiency of the power generation process.